1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine and a printer, and more particularly to an image forming apparatus using a contact charging device.
2. Description of the Related Art
Electrophotographic image forming apparatuses employ charging devices for uniformly charging surfaces of electrostatic latent image carriers such as photosensitive drums. The charging devices can be basically classified into corona charging devices and contact charging devices. As the known contact charging devices, there are charging devices of fixed brush charging devices, rotatable brush charging devices, roller charging devices, blade charging devices, magnetic brush charging devices and others.
However, in the electrophotographic image forming apparatus, toner fragments other than the toner may be produced in the developing devices and others. The toner fragments are charged at the polarity opposite to the regular or proper charge polarity of the toner. The toner fragments may adhere to a surface of an electrostatic latent image carrier, and may not be removed sufficiently by a cleaning device. Also, shaved powder or the like of the electrostatic latent image carrier having the polarity opposite to the regular charge polarity of the toner may be produced. The shaved powder or the like may not be removed by the cleaning device.
For example, if a contact charging device is used as a device for charging the surface of the electrostatic latent image carrier prior to formation of the electrostatic latent image, these toner fragments, shaved powder and others having the opposite polarity (also referred to as counter charged toner) may adhere to the charging device, which results in defective charging and thus image noises called as striped noises.
As measures for solving the foregoing problem, the Japanese Examined Patent Publication No. 63-43750 (43750/1988) has taught a technique in which a voltage, which includes a D.C. component of the same polarity as the charge polarity of the electrostatic latent image carrier, is applied to the charging device in the charging operation, and a voltage, which includes a D.C. component having the polarity opposite to the charge polarity of the electrostatic latent image carrier, is applied to the electrostatic latent image carrier when the charging operation is not carried out, so that stain by the fine powder adhered to the charging device may be removed.
However, in the foregoing prior art, the stain by the fine powder adhered to the contact charging device is removed by charging the surface of the electrostatic latent image carrier at the polarity opposite to the regular charge polarity, so that the charge of the opposite polarity is accumulated on and in the surface of the electrostatic latent image carrier. As a result, this charge may cancel the regular charge of the regular polarity which is used for the regular charging of the surface of the electrostatic latent image carrier in a subsequent image forming step, resulting in reduction of the amount of the charge and/or irregular charging.